The spine includes a series of joints routinely called motion segment units, which is the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. Each component of the motion segment unit contributes to the mechanical stability of the joint.
Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, or laminectomy.
Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. The purpose of fusion or rigid stabilization is the immobilization of a motion segment unit. Thus, fusion or rigid stabilization of one or more motion segment units may be an important element of a surgical procedure in certain cases (i.e., injuries, deformities, tumors, etc.). In other cases it is a complementary element (i.e., fusion performed due to degeneration).
One current spinal surgical technique typically involves fusing one or more unstable motion segment units and possibly, the removal of ligaments, bone, disc, or combinations thereof included in the unstable motion segment unit or units prior to fusing. However, fusion often involves several disadvantages. For example, the fusing process results in a permanent or rigid internal fixation of all or part of the intervertebral joints and usually involves metallic rods, plates, and the like for stabilization. Such systems are intended to rigidly immobilize the motion segment unit to promote fusion within that motion segment unit resulting in a loss of mobility.
Fusion also causes the mobility of the motion segment to be transferred to other motion segments of the spine. The added stresses transferred to motion segments neighboring or nearby the fused segment can cause or accelerate degeneration of those segments. Another disadvantage of fusion is that it is an irreversible procedure.
Spinal fixation systems using orthopedic rods as described above provide a relatively rigid construct. Another type of spinal treatment technique offers dynamic stabilization for the spine. One example of a dynamic stabilization system is provided by the assignee of this invention under the trademark DYNESYS® as generally described in European Patent Application No. 0669109A1 which is hereby incorporated by reference in its entirety. Such a dynamic stabilization system utilizes pedicle screws installed into appropriate locations in adjacent vertebrae. Flexible materials in conjunction with the pedicle screws are used rather than rigid orthopedic rods or bone grafts alone as an adjunct fusion. A tubular spacer is positioned between the pedicle screws on adjacent vertebrae with a tensioned cord passing through the central lumen of the spacer. Dynamic stabilization systems of this type bring the lumbar vertebrae into a desired anatomical position while stabilizing the effective segments and without irreversible fusion.
Fusion procedures can be improved by modifying the load sharing characteristics of the treated spine and one technique is to allow more of a physiologic loading between pedicular fixation and anterior column support. Additionally, a device and associated method that precludes or at least delays the need for fusion for all but the most advanced degeneration of a motion segment, particularly if such a device would allow close to normal motion and pain relief is desirable. Moreover, utilization of minimally invasive surgical procedures and techniques to install such devices is also a highly desirable objective to enhance and promote patient healing and recovery.